cDNAs of two non-allelic sucrose synthase genes in maize: cloning, expression, characterization and molecular mapping of the sucrose synthase-2 gene.

cDNA clones of the two non-allelic sucrose synthase (Ss) genes, Ss2 and Sh, have been isolated from λgt11 expression libraries derived from immature kernel poly(A)(+) RNA of sh-deletion and Sh/Sh genotypes of maize respectively. Recombinant clones containing the longest Ss2 and Sh cDNA inserts, each of approximately 2.5 kb size, were characterized and comparatively analyzed. Although the Sh cDNA insert expresses as a sucrose synthase-1 (SS1) β-galactosidase fusion protein (∼ 200 kD) in λ lysogens, the Ss2 cDNA failed to form such a chimeric protein and instead showed a ∼ 70 kD SS2 polypeptide. The Ss2 and Sh cDNAs as hybridization probes on RNA blots of immature kernels detected a larger Ss2 transcript (∼ 2900 b) than the Sh transcript (∼ 2750 b). Because SS1 and SS2 protein subunits are known to be of identical size, the significance of difference in transcript size is not apparent. A comparative restriction enzyme mapping of the two cDNA clones and a genomic Ss2 clone show sequence diversity over the entire lengths of Ss2 and Sh clones. Interestingly, restriction endonuclease sites around the 3' ends are more conserved than the 5' ends of these two genes. Genetic data indicate that the Ss2 locus is on chromosome 9 and molecular mapping using the Ss2 cDNA clone on recombinant inbred lines and B-A translocations stocks suggest that Ss2 is about 20 map units away from the Wx locus on 9L.